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More Output Current Swing

By Robert J. Widlar

High Current Output Buffer with a pair of complementary transistors.

Because almost all monolithic amplifiers use class-B output stages, they have
good loaded output voltage swings, delivering ±10V at 5 mA with ±15V supplies. Demanding
much more current from the integrated circuit would require, for one, that the output
transistors be made considerably larger. In addition, the increased dissipation
could give rise to troublesome thermal gradients on the chip as well as excessive
package heating in high-temperature applications. It is therefore advisable to use
an external buffer when large output currents are needed.

Figure 1. High Current Output Buffer

A simple way of accomplishing this is shown in Figure 1. A pair of complementary
transistors are used on the output of the LM101 to get the increased current swing.
Although this circuit does have a dead zone, it can be neglected at frequencies
below 100 Hz because of the high gain of the amplifier. R1 is included to eliminate
parasitic oscillations from the output transistors. In addition, adequate bypassing
should be used on the collectors of the output transistors to insure that the output
signal is not coupled back into the amplifier. This circuit does not have current
limiting, but it can be added by putting 50Ω resistors in series with the collectors
of Q1 and Q2.